VISCOMETRIC CHARACTERIZATION OF POLY(OXYETHYLENE)-BLOCK-POLY(OXYPROPYLENE)-BLOCK-POLY(OXYETHYLENE) IN AQUEOUS-SOLUTION

Viscometric measurements were carried out to characterize poly(oxyethylene)-block-poly(oxypropylene)-block-poly(oxyethylene) (POE-POP-POE) in aqueous solution. The results were discussed in terms of a core-shell model of intramolecular phase separation which is based on i) the selectivity of the solvent water, and ii) the incompatibility of the POE and POP blocks. The interaction between the two blocks was analysed by vapour pressure osmometry and vapour sorption investigations. The results are: 1. The two homopolymer blocks (POE and POP) forming the triblock copolymer are incompatible with each other as vapour pressure osmometry and vapour sorption measurements using ethylbenzene as solvent have shown. This incompatibility of the blocks as well as the selectivity of the solvent water with respect to the different blocks causes the intramolecular (and intermolecular) phase separation of the copolymers in aqueous solution. 2. The viscometric data of the aqueous block copolymer solutions can be discussed in terms of the generally accepted core-shell model. This model for the structure of the self-organized molecules includes POP gel core ("melt") surrounded by solvated POE shell. A quantitative analysis according to this simplified view shows that the hydrodynamic volume of the copolymer molecules is determined by the POE blocks. In the temperature range curly theta > 30-degrees-C, the influence of the POP block on the molecular dimensions is negligible, whereas at curly theta < 30-degrees-C the contributions of the POP and POE to the molecular volume are comparable. 3. Kuhn's statistical segment length of POE in the triblock molecules comprises about 4-5 monomeric units. The theta temperature of the triblock copolymer molecules is nearly identical to that of pure POE ((369 +/- 3)K).